The gonadotrophins are a group of heterodimeric glycoprotein hormones which regulate gonadal function in the male and female. They include follicle stimulating hormone (FSH), luteinising hormone (LH) and chorionic gonadotrophin (CG).
FSH is naturally secreted by the anterior pituitary gland and functions to support follicular development and ovulation. FSH comprises a 92 amino acid alpha sub-unit, also common to the other glycoprotein hormones LH and CG, and a 111 amino acid beta sub-unit unique to FSH that confers the biological specificity of the hormone (Pierce and Parsons, 1981). Each sub-unit is post translationally modified by the addition of complex carbohydrate residues. Both subunits carry 2 sites for N-linked glycan attachment, the alpha sub-unit at amino acids 52 and 78 and the beta sub-unit at amino acid residues 7 and 24 (Rathnam and Saxena, 1975, Saxena and Rathnam, 1976). FSH is thus glycosylated to about 30% by mass (Dias and Van Roey. 2001. Fox et al. 2001).
FSH purified from post-menopausal human urine has been used for many years in infertility treatment; both to promote ovulation in natural reproduction and to provide oocytes for assisted reproduction technologies. Two recombinant versions of FSH, GONAL-F® (Serono) and PUREGON™ (Organon) became available in the mid-1990's. These are both expressed in Chinese hamster ovary (CHO) cells (Howles, 1996).
There is considerable heterogeneity associated with FSH preparations which relates to differences in the amounts of various isoforms present. Individual FSH isoforms exhibit identical amino acid sequences but differ in the extent to which they are post-translationally modified; particular isoforms are characterised by heterogeneity of the carbohydrate branch structures and differing amounts of sialic acid (a terminal sugar) incorporation, both of which appear to influence the specific isoform bioactivity.
Glycosylation of natural FSH is highly complex. The glycans in naturally derived pituitary FSH can contain a wide range of structures that can include combinations of bi-, tri- and tetra-antennary glycans (Pierce and Parsons, 1981. Ryan et al., 1987. Baenziger and Green, 1988). The glycans can carry further modifications: core fucosylation, bisecting glucosamine, chains extended with acetyl lactosamine, partial or complete sialylation, sialylation with α2,3 and α2,6 linkages, and sulphated galactosamine substituted for galactose (Dalpathado et al., 2006). Furthermore, there are differences between the distributions of glycan structures at the individual glycosylation sites. A comparable level of glycan complexity has been found in FSH derived from the serum of individuals and from the urine of post-menopausal women (Wide et al., 2007).
The glycosylation of recombinant FSH products reflects the range of glycosyl-transferases present in the host cell line. Existing rFSH products are derived from engineered Chinese hamster ovary cells (CHO cells). The range of glycan modifications in CHO derived rFSH are more limited than those found on the natural products, derived either from pituitary extracts or urine. Examples of the reduced glycan heterogeneity found in CHO derived rFSH include a lack of bisecting glucosamine and a reduced content of core fucosylation and acetyl lactosamine extensions (Hard et al., 1990). In addition, CHO cells are only able to add sialic acid using the α2,3 linkage (Kagawa et al, 1988, Takeuchi et al, 1988, Svensson et al., 1990). This is different from naturally produced FSH which contains glycans with a mixture of α2,3 and α2,6-linked sialic acid.
It has been demonstrated that a recombinant FSH preparation (Organon) differs in the amounts of FSH with an isoelectric point (pI) of below 4 (considered the acidic isoforms) when compared to pituitary, serum or post-menopausal urine FSH (Ulloa-Aguirre et al. 1995). The amount of acidic isoforms in the urinary preparations was much higher as compared to the recombinant products, GONAL-F® (Serono) and PUREGON™ (Organon) (Andersen et al. 2004). This must reflect a lower molar content of sialic acid in the rFSH since the content of negatively-charged glycan modified with sulphate is low in FSH. The lower sialic acid content, compared to natural FSH, is a feature of both commercially available FSH products and therefore must reflect a limitation in the manufacturing process (Bassett and Driebergen, 2005).
There is a large body of scientific work which analyses and tries to explain the variations in FSH glycosylation between individuals and changes over the course of an ovulation cycle. One of the major discussions relates to the observation that FSH concentration and sialic acid content both decrease during the pre-ovulatory phase of the cycle. The decreased sialic acid content results in a more basic FSH which is both cleared more rapidly and, in vitro at least, is more potent at the target receptor (Zambrano et al. 1996). The question as to the biological relevance of these changes and how they may be involved in selecting the dominant follicle remains unresolved (reviewed by Ulloa-Aguirre, 2003).
The circulatory life-time of FSH has been documented for materials from a variety of sources. Some of these materials have been fractionated on the basis of overall molecular charge, as characterised by their pI, in which more acid equates to a higher negative charge. As previously stated the major contributor to overall molecular charge is the total sialic content of each FSH molecule. For instance, rFSH (Organon) has a sialic acid content of around 8 mol/mol, whereas urine-derived FSH has a higher sialic acid content (de Leeuw et al. 1996). The corresponding plasma clearance rates in the rat are 0.34 and 0.14 ml/min (Ulloa-Aguirre et al. 2003). In another example where a sample of recombinant FSH was split into high and low pI fractions, the in vivo potency of the high pI (lower sialic acid content) fraction was decreased and it had a shorter plasma half-life (D'Antonio et al. 1999). It has also been reported that the more basic FSH circulating during the later stages of the ovulation cycle is due to the down-regulation of α2,3 sialyl-transferase in the anterior pituitary which is caused by increasing levels of estradiol (Damian-Matsumara et al. 1999. Ulloa-Aguirre et al. 2001). Results for the α2,6 sialyl-transferase have not been reported.
The total sialic acid content of FSH and rFSH is not directly comparable since sialic acids are commonly linked in two ways. Pituitary/serum/urinary FSH contain both α2,3 and α2,6-linked sialic acid, with a predominance of the former. However, CHO cell derived recombinants only contain α2,3 (Kagawa et al, 1988, Takeuchi et al, 1988, Svensson et al., 1990). This is another difference between natural and current recombinant products in addition to the lower overall sialic acid content of the latter.
CHO cells are commonly used for the production of pharmaceutical human recombinant proteins. Structural analysis has identified that sialic acid is exclusively attached by a α2,3-linkage. (Kagawa et al, 1988, Takeuchi et al, 1988, Svensson et al., 1990). Many human glycoproteins contain a mixture of both α2,3- and α2,6-linkages. Therefore recombinant proteins expressed using the CHO system will differ from their natural counterparts in their type of terminal sialic acid linkages. This is an important consideration in the production of biologicals for pharmaceutical use since the carbohydrate moieties may contribute to the pharmacological attributes of the molecule.